Mammalian tissues are dependent upon the continuous supply of metabolic energy (such as ATP and phosphocreatine) in order to perform its various vital activities (biosynthesis, ion transport, etc.). The main source of energy to be utilized by most body tissues is glucose transported by the blood stream together with oxygen (FIG. 1A). The oxygen and glucose pass from the vascular compartment to the cells through the extracellular space. The O.sub.2 molecule serves as a terminal electron acceptor in the respiratory chain located in the inner membrane of the mitochondrion. Under normal O.sub.2 supply, the complicated enzymatic processes of glucose breakdown to CO.sub.2 and H.sub.2 O could be divided into two major steps, namely, the anaerobic (not dependent on O.sub.2), followed by the aerobic step (O.sub.2 dependent) which occurs in the mitochondrion. See FIG. 1B. Changes in tissue energy metabolism may have a transient nature or may be permanent. Therefore, in order to assess the tissue energy state, it is necessary to monitor the events continuously in real-time mode.
Any change in brain electrical activity will result in an activation of the ion pumps in an effort to restore normal ion distribution. A decrease (ischemia) or increase (hyperoxia) in the O.sub.2 supply to the brain will affect the balance between energy supply and demand and may result in a pathological state. In the pioneering work described by Chance and Williams (Chance B. and Williams, G. R., "Respiratory enzymes in oxidative phosphorylation. I- Kinetics of oxygen utilization", J. Biol. Chem. 217, 383-393, 1955), which is incorporated by reference as if fully set forth herein, several metabolic states for the isolated mitochondria depending upon the availability of O.sub.2, substrate and ADP, were defined. The "resting state", state 4, exhibited high O.sub.2 and substrate levels, with the limiting factor being the ADP. The active state, state 3, could be induced by the addition of ADP to the resting mitochondria. This will lead to the formation of more ATP and to the oxidation of the various electron carriers in the mitochondria. In state 3, which is the active state during which O.sub.2 consumption is increased, cerebral blood flow will increase in order to compensate for the increased O.sub.2 consumption (Mayevsky, A. and Weiss, H. R., Cerebral blood flow and oxygen consumption in cortical spreading depression, J. CBF and Metabol. 11: 829-836 (1991), which is incorporated by reference as if fully set forth herein). While in state 4, 99% of the Nicotine Adenine Dinucleotide (NADH) will be in the reduced form, in state 3 only about 50% of the NADH will be oxidized. The "resting" brain in vivo is probably between state 4 and state 3 (Mayevsky, A., Brain energy metabolism of the conscious rat exposed to various physiological and pathological situations, Brain Res. 113: 327-338 (1976), which is incorporated by reference as if fully set forth herein).
There is a direct coupling between energy metabolism of the cellular compartment and the blood flow in the microcirculation of the same tissue. In a normal tissue any change in O.sub.2 supply (decrease or increase) will be compensated by a change in blood flow (increase or decrease, respectively). By this mechanism, O.sub.2 supply will remain constant if O.sub.2 consumption was not affected. In cases when O.sub.2 consumption is stimulated by activation of ion pumping activity or biosynthesis, the blood flow will be stimulated in order to supply more O.sub.2. See FIG. 1B.
A change in blood flow and volume (i.e., in intra vascular velocity and concentration of red blood corpuscles) may change the apparent energy state. See FIG. 1A. Knowledge of the blood supply via microcirculatory blood flow (MBF) by itself or of the mitochondrial redox state of Nicotine Adenine Dinucleotide (NADH), by itself, is of limited value. It will not provide reliable information due to various unclear responses to pathological events such as hypoxia, ischemia or brain stimulation (epileptic activity or spreading depression).
The effects of ischemia on the CBF of the brain were tested in the Mongolian gerbil in which partial or complete ischemia can be induced by unilateral or bilateral carotid artery occlusions (Mayevsky, A. Level of ischemia and brain functions in the Mongolian gerbil in vivo, Brain Res., 524: 1-9 (1990), which is incorporated by reference as if fully set forth herein). 38 occlusions (unilateral or bilateral) were performed in a group of 28 gerbils. The level of ischemia as evaluated by the Laser Doppler flowmeter was compared to the intramitochondrial change in the NADH redox state. The two parameters were normalized to the rage of 0 to 100% and the results, which are presented in FIG. 1D, show a significant correlation between the decrease in flow (increase ischemia) and the increase in NADH levels (R=0.73, p&lt;0.001, N=38). Note the significant scatter in the data which demonstrates another shortcoming of using two separate instruments.
The use of these two parameters enables one to quantify the level of ischemia and tissue O.sub.2 deficiency and overcomes the inability to quantify those two parameters in absolute units (such as ml CBF per unit time). In order to reliably assess tissue energy state, it is necessary to monitor both MBF and NADH continuously from the same volume element of tissue.
The present invention relates to an apparatus for monitoring the viability of body tissue. In particular, the present invention relates to an apparatus or device for measuring two or more parameters indicative of the function of the tissue, storing and retrieving said information, enabling long term monitoring.
More particularly, the present invention relates to a single probe device that measures parameters indicative of the function of the tissue in the identical volume element of the tissue (determining their unique ratios) storing and retrieving said information, to enable long term monitoring.
Most particularly, the main two parameters to be monitored from the tissue are microcirculatory blood flow (MBF) and mitochondrial redox state (NADH fluorescence), as these provide considerable tissue viability information. Until now MBF was measured by Laser Doppler flowmetry while mitochondrial redox state was evaluated by monitoring oxidation reduction state of NADH (Mayevsky, A., Brain NADH redox state monitored in vivo by fiber optic surface fluorometry, Brain Res. Rev. 7: 49-68, (1984), which is incorporated by reference as if fully set forth herein). These measurements were done by using two separate instruments utilizing specific optical properties of the red blood cells and the mitochondrial enzymatic activities. In the present application it is suggested to monitor tissue blood flow (MBF) and intramitochondrial redox state (NADH) of the same volume element of tissue, at the same time, using a single probe, with two or more analyzers of the return signals.
The prior art teaches a wide variety of apparatus/devices which monitor various parameters reflecting the viability of the tissue. For example, U.S. Pat. No. 4,703,758 teaches the use of an apparatus to monitor blood flow by using a light source to emit a beam of light and a light detector that measures the light received. This provides the value of the intensity of the transmitted light, which inter alia depends upon the blood flow in the path of the light.
U.S. Pat. No. 4,945,896 teaches the use of a multiprobe sensor, using independent microelectrodes implanted inside the brain tissue, for measuring various parameters indicative of the function of the brain including a laser doppler flow probe for measuring cerebral blood flow, and a probe for monitoring redox state (NADH). These probes can be put in sequentially, i.e., one after another in the same housing, or they both can be in together, side by side.
These devices suffer from a major drawback. Tissue viability is not merely a reflection of various values of parameters measured at different times in one place, or different places at one time. Rather, the values of blood flow and redox state (NADH) must be monitored simultaneously on the identical volume element of tissue. The complex biochemical mechanisms that determine tissue viability are such that short time deviations between measurement or short distances between points of measurement can provide inaccurate or even misleading information. For example, illuminating an organ in the UV (366 nm) causes fluorescence (at 440 to 480 nm). The measured intensity of this fluorescence reflects the oxidation-reduction state ((NADH)/(NAD ratio) of this organ. However, variation in intra-tissue concentration of red blood corpuscles effects the measurement.
In particular, a reduction of these red blood corpuscles causes an increase in fluorescence, generating a false indication of the true oxidation reduction state of the organ. Hence, U.S. Pat. No. 4,449,535 teaches the use of simultaneously monitoring the concentration of red corpuscles, by illuminating at a red wavelength (720 nm) at the same time, at the same spot or place as the UV, and measuring the variation in intensity of the reflected red radiation as well as the fluorescence at 440-480 nm, the former being representative of the intra-tissue concentration of red blood corpuscles.
This approach involves concentrating both the UV and red pulses onto a single point, an optical fiber to convey both pulses in one direction, and fluorescence and reflected radiation in the other, and two photoelectric receivers for detecting the respective wavelengths. The quality of the information is limited both by the possibility that the red radiation is a perturbation to the tissue that can affect its fluorescence (and vice versa) and by the need to prevent interference between the two output signals (e.g., interference reflections being picked up by the receivers). The use of red radiation to correct for blood hemodynamic artifacts in the NADH signal introduces inaccuracies into the measurements due to differences in absorption volume.
Furthermore, because the different radiations penetrate the tissue to different extents, effectively two different elements of tissue volume are being probed, even if both radiations fall on same spot or place. Also, as two different wavelengths and two different sources of radiation are used, there is a relative instability associated with the reflectometer and fluorimeter readings, especially with time sharing compared to using a single source, single wavelength. This could limit its use for monitoring to minutes or hours as opposed to days.
Thus, for the above reasons, among others, there is a need for a single probe apparatus or device which can provide high quality information, which is stable, i.e., without time sharing between Fluorimeter and Reflectometer about viability of tissue by monitoring at least two or more parameters at the same time, on the same tissue volume, without possible perturbations or interferences between the different input or output signals, respectively, distorting the information.